Prometheus 注册中心Eureka服务发现原理

概述

Eureka服务发现协议允许使用Eureka Rest API检索出Prometheus需要监控的targets，Prometheus会定时周期性的从Eureka调用Eureka Rest API，并将每个应用实例创建出一个target。

【资料图】

Eureka服务发现协议支持对如下元标签进行relabeling：

__meta_eureka_app_name: the name of the app__meta_eureka_app_instance_id: the ID of the app instance__meta_eureka_app_instance_hostname: the hostname of the instance__meta_eureka_app_instance_homepage_url: the homepage url of the app instance__meta_eureka_app_instance_statuspage_url: the status page url of the app instance__meta_eureka_app_instance_healthcheck_url: the health check url of the app instance__meta_eureka_app_instance_ip_addr: the IP address of the app instance__meta_eureka_app_instance_vip_address: the VIP address of the app instance__meta_eureka_app_instance_secure_vip_address: the secure VIP address of the app instance__meta_eureka_app_instance_status: the status of the app instance__meta_eureka_app_instance_port: the port of the app instance__meta_eureka_app_instance_port_enabled: the port enabled of the app instance__meta_eureka_app_instance_secure_port: the secure port address of the app instance__meta_eureka_app_instance_secure_port_enabled: the secure port of the app instance__meta_eureka_app_instance_country_id: the country ID of the app instance__meta_eureka_app_instance_metadata_: app instance metadata__meta_eureka_app_instance_datacenterinfo_name: the datacenter name of the app instance__meta_eureka_app_instance_datacenterinfo_: the datacenter metadata

eureka_sd_configs常见配置如下：

- job_name: "eureka"  eureka_sd_configs:    - server: http://localhost:8761/eureka #eureka server地址      refresh_interval: 1m #刷新间隔，默认30s

eureka_sd_configs官网支持主要配置如下：

server: basic_auth:  [ username:  ]  [ password:  ]  [ password_file:  ]# Configures the scrape request"s TLS settings.tls_config:  [  ]# Optional proxy URL.[ proxy_url:  ]# Configure whether HTTP requests follow HTTP 3xx redirects.[ follow_redirects:  | default = true ]# Refresh interval to re-read the app instance list.[ refresh_interval:  | default = 30s ]

Eureka协议实现

基于Eureka服务发现协议核心逻辑都封装在discovery/eureka.go的func (d *Discovery) refresh(ctx context.Context) ([]*targetgroup.Group, error)方法中：

func (d *Discovery) refresh(ctx context.Context) ([]*targetgroup.Group, error) { // 通过Eureka REST API接口从eureka拉取元数据：http://ip:port/eureka/apps apps, err := fetchApps(ctx, d.server, d.client) if err != nil {  return nil, err } tg := &targetgroup.Group{  Source: "eureka", } for _, app := range apps.Applications {//遍历app        // targetsForApp()方法将app下每个instance部分转成target  targets := targetsForApp(&app)        //解析的采集点合入一起  tg.Targets = append(tg.Targets, targets...) } return []*targetgroup.Group{tg}, nil}

refresh方法主要有两个流程：

1、fetchApps()：从eureka-server的/eureka/apps接口拉取注册服务信息；

2、targetsForApp()：遍历app下instance，将每个instance解析出一个target，并添加一堆元标签数据。

如下示例从eureka-server的/eureka/apps接口拉取的注册服务信息：

    1    UP_1_            SERVICE-PROVIDER-01                    localhost:service-provider-01:8001            192.168.3.121            SERVICE-PROVIDER-01            192.168.3.121            UP            UNKNOWN            8001            443            1                            MyOwn                                        30                90                1629385562130                1629385682050                0                1629385562132                                        8001                true                8080                        http://192.168.3.121:8001/            http://192.168.3.121:8001/actuator/info            http://192.168.3.121:8001/actuator/health            service-provider-01            service-provider-01            false            1629385562132            1629385562039            ADDED

instance信息会被解析成采集点target：

func targetsForApp(app *Application) []model.LabelSet { targets := make([]model.LabelSet, 0, len(app.Instances)) // Gather info about the app"s "instances". Each instance is considered a task. for _, t := range app.Instances {  var targetAddress string        // __address__取值方式：instance.hostname和port，没有port则默认port=80  if t.Port != nil {   targetAddress = net.JoinHostPort(t.HostName, strconv.Itoa(t.Port.Port))  } else {   targetAddress = net.JoinHostPort(t.HostName, "80")  }  target := model.LabelSet{   model.AddressLabel:  lv(targetAddress),   model.InstanceLabel: lv(t.InstanceID),   appNameLabel:                     lv(app.Name),   appInstanceHostNameLabel:         lv(t.HostName),   appInstanceHomePageURLLabel:      lv(t.HomePageURL),   appInstanceStatusPageURLLabel:    lv(t.StatusPageURL),   appInstanceHealthCheckURLLabel:   lv(t.HealthCheckURL),   appInstanceIPAddrLabel:           lv(t.IPAddr),   appInstanceVipAddressLabel:       lv(t.VipAddress),   appInstanceSecureVipAddressLabel: lv(t.SecureVipAddress),   appInstanceStatusLabel:           lv(t.Status),   appInstanceCountryIDLabel:        lv(strconv.Itoa(t.CountryID)),   appInstanceIDLabel:               lv(t.InstanceID),  }  if t.Port != nil {   target[appInstancePortLabel] = lv(strconv.Itoa(t.Port.Port))   target[appInstancePortEnabledLabel] = lv(strconv.FormatBool(t.Port.Enabled))  }  if t.SecurePort != nil {   target[appInstanceSecurePortLabel] = lv(strconv.Itoa(t.SecurePort.Port))   target[appInstanceSecurePortEnabledLabel] = lv(strconv.FormatBool(t.SecurePort.Enabled))  }  if t.DataCenterInfo != nil {   target[appInstanceDataCenterInfoNameLabel] = lv(t.DataCenterInfo.Name)   if t.DataCenterInfo.Metadata != nil {    for _, m := range t.DataCenterInfo.Metadata.Items {     ln := strutil.SanitizeLabelName(m.XMLName.Local)     target[model.LabelName(appInstanceDataCenterInfoMetadataPrefix+ln)] = lv(m.Content)    }   }  }  if t.Metadata != nil {   for _, m := range t.Metadata.Items {                // prometheus label只支持[^a-zA-Z0-9_]字符，其它非法字符都会被替换成下划线_    ln := strutil.SanitizeLabelName(m.XMLName.Local)    target[model.LabelName(appInstanceMetadataPrefix+ln)] = lv(m.Content)   }  }  targets = append(targets, target) } return targets}

解析比较简单，就不再分析，解析后的标签数据如下图：

标签中有两个特别说明下：

1、__address__：这个取值instance.hostname和port(默认80)，所以要注意注册到eureka上的hostname准确性，不然可能无法抓取；

2、metadata-map数据会被转成__meta_eureka_app_instance_metadata_格式标签，prometheus进行relabeling一般操作metadata-map，可以自定义metric_path、抓取端口等；

3、prometheus的label只支持[a-zA-Z0-9_]，其它非法字符都会被转换成下划线，具体参加：strutil.SanitizeLabelName(m.XMLName.Local)；但是eureka的metadata-map标签含有下划线时，注册到eureka-server上变成双下划线，如下配置:

eureka:  instance:    metadata-map:      scrape_enable: true      scrape.port: 8080

通过/eureka/apps获取如下：

总结

基于Eureka服务发现原理如下图：

基于eureka_sd_configs服务发现协议配置创建Discoverer，并通过协程运行Discoverer.Run方法，Eureka服务发现核心逻辑封装discovery/eureka.go的func (d *Discovery) refresh(ctx context.Context) ([]*targetgroup.Group, error)方法中。

refresh方法中主要调用两个方法：

1、fetchApps：定时周期从Eureka Server的/eureka/apps接口拉取注册上来的服务元数据信息；

2、targetsForApp：解析上步骤拉取的元数据信息，遍历app下的instance，将每个instance解析成target，并将其它元数据信息转换成target元标签可以用于relabel_configs操作

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